The present invention relates to a semiconductor light emitting diode (LED) array. In particular, the present invention relates to a semiconductor LED array which is adjustable by a user for the selection of a desired color temperature. Also, the present invention relates to a method of selecting a desired color temperature from an array of LEDs.
The color temperature of light is typically measured in degrees Kelvin (K). This measurement system was first adapted to measure the temperature of stars. With this color temperature scale, the colder the light, the higher the degrees K, i.e., the hotter the star, the bluer the light output. This temperature scale is also used to measure the light output of other light sources, such as incandescent bulbs, fluorescent lamps and LEDs, to name a few.
To provide the proper contrast for items in an individual""s viewing environment, it is desirable to have a white light output from a light source. The use of incandescent bulbs and flourescent lamps have effectively provided such a white light or near white light output. However, there are significant drawbacks to the use of these types of light sources for illumination, such as fragility of the lights themselves and their relatively short lifespan. With incandescent bulbs, for example, their output color temperature will shift toward the red end of the spectrum with a drop in line voltage. Also, changes in the output color temperature due to bulb aging are particularly problematic in color photography or cinematography applications where changes in color temperature due to aging over a very short period (i.e., 48 hours of operation) necessitate the frequent changing of very expensive bulbs.
Because of the drawbacks in the use of incandescent and fluorescent lights, the use of LEDs for illumination has become increasingly popular. However, because LEDs use semiconductor principles of operation to produce light, their light output is typically along a narrow wavelength band, i.e., a single color output. Recent advances, however, have resulted in LEDs which produce a near white light output.
Presently, there are two methods utilized to output white light from LEDs. The first method uses triads of red, green, and blue LEDs. This first method requires a very careful balancing of the brightness of each of the three colors to obtain a white light output. Once the white light output is established, an extremely fine adjustment is then required to obtain the desired color temperature. This is because variations within a fraction of a percent in the intensity of any one color LED will result in a perceptible change in the overall output color temperature of the white light. Further, as the light output of the LEDs vary with age, the ambient temperature changes, and the drive current supplied to the LEDs varies even slightly, the color temperature of the white light will exhibit unwanted fluctuations.
One method for dealing with this problem is to adjust the LEDs for as pure a white light output as possible, and then correct for color temperature using tinted filters. This method ameliorates the color shift problem, but results in significant light losses.
The second method for generating white light is to use a special type of LED which produces a white light output. This special LED produces a white light output by coating the emitting surface of a high intensity blue LED with a phosphor which emits yellow light. The yellow light is emitted as a secondary emission as a result of the phosphor being excited by the photons from the blue LED junction. The spectral output of these devices shows a very high output at the wavelengths in the blue end of the spectrum and a moderate spike in the output at the wavelengths near the yellow portion of the spectrum. Thus, the overall output of the device is a white light with a relatively high color temperature. Such high temperature white LEDs are available from Nichia Chemical Corporation. These white LEDs are available over a range of color temperatures from 5000 deg. K to 8500 deg. K. To obtain lower color temperatures so as to approximate the light from an incandescent lamp, i.e., a color temperature of about 3600 deg. K, a color correcting filter with its attendant light losses must be used.
Therefore, there remains a need for a white light LED which is simple and can be easily adjusted to produce a white light of a desired color temperature.
The present invention provides an LED arrangement which produces a color temperature adjustable white light. The LED arrangement includes one or more white LEDs, a first drive circuit operable to supply a first drive current to the one or more white LEDs such that a white light is output at a desired intensity. The LED arrangement also includes one or more colored LEDs arranged such that a colored light output from the one or more colored LEDs combines with the white light to produce a resultant light having a desired color temperature. A second drive circuit is provided to supply a second drive current to the one or more colored LEDs such that the colored light is output at a desired intensity. The intensity of the colored light output from the one or more colored LEDs is adjustable such that the color temperature of the resultant light is adjustable.
In the preferred embodiments, the colored LEDs are either amber LEDs, or a combination of red and yellow LEDs. The LEDs used can be either discrete LEDs or xe2x80x9cchip-on-boardxe2x80x9d LEDs.
With this arrangement of LEDs and driver circuits, the color temperature of a white LED can be effectively adjusted without the output color temperature being sensitive to aging, fluctuations in ambient temperature and changes in drive current. The ability of the present invention to effectively adjust the color temperature of the resultant light to reduce the effects aging, fluctuations in ambient temperature and changes in drive current is a result of utilizing the additive properties of light, as opposed to using subtractive properties, such as color filters and their attendant light losses.
Further, the LED arrangement of the present invention allows for the adjustment of the color temperature over a wide range and achieves the desired color temperature even when the intensity of the light varies by several percent in either direction without causing a perceptible change in color temperature.